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Dames & Moore, 1999 - USDA Forest Service

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the vicinity with differences probably caused by dilution. Attenuation of metal concentrations appears to<br />

be caused principally by precipitation of aluminum hydroxides. Attenuation by adsorption in soils is not<br />

apparent. Seeps SP-24 and SP-25 may be influenced by the lagoon area.<br />

Seep SP-8 flows from the east waste rock pile and then flows partly overland and subsurface, re-emerging<br />

at SP-19 (Figure 6.1-3a). The differences for magnesium (which acts as a conservative ion)<br />

concentrations between these two points are consistent with dilution by a non-buffering water source<br />

(e.g., snow melt). The water is probably well-oxidized at SP-8 as shown by low iron concentrations.<br />

MMTEQA2 predicts that aluminum minerals would be expected to precipitate. As pH was not observed<br />

to change between the two points (4.6 S.U.), and aluminum concentrations decreased in proportion to<br />

conservative ions, no additional precipitation appears to occur between these points.. I<br />

Significant metal attenuation between SP-8 and SP-19 does not appear to occur.<br />

Seeps SP-19 to SP-1OE and SP-1OW<br />

Seep SP-19 flows' from the western portion of tailings pile 1 into the Copper Creek diversion near the<br />

river sauna (Figure 6.1-3a). Seeps SP- IOE and SP- I OW flow from the south bank of Railroad Creek<br />

north of the river sauna. SP-IOW closely resembles SP-19 and MINTEQA2 predicted similar controlling<br />

conditions. The pH of the water appears to be controlled by aluminum minerals. Conservative ions<br />

indicate that SP-19 and SP-1OW are nearly identical. Zinc and copper concentrations were, however,<br />

lower than other seeps nearby. This implies either that the water mixed with a source containing<br />

comparable sulfate, etc. and very low zinc and copper, or that zinc and copper were adsorbed by contact<br />

with soil.<br />

Seep SP-IOE indicated 20 percent less sulfate, 80 percent less zinc and 64 percent less copper than SP-<br />

1 OW in May 1997. However, pH was 3.3 for SP-1 OE compared to 4.5 for SP- I OW. MMTEQA2 indicate<br />

that SP-IOE water is in equilibrium with iron hydroxide as a result of 14 mg Fe/L. Field measured Eh for<br />

both SP-1OW and SP- 1OE were comparable and indicated that the waters are well-oxidized.<br />

The difference between the SP-IOW and SP-1OE waters could be explained by the mixing of a water<br />

containing high iron concentrations with water of the SP-IOW. If the water contained reduced iron, it<br />

would oxidize and hydrolyze upon emergence (equations 6-5 and 6-6 presented earlier in this section),<br />

buffering pH near 3 and causing ferric hydroxide to precipitate. Copper and zinc would co-precipitate.<br />

The iron-rich water would have to contain less sulfate than SP-IOW to produce the lower sulfate in SP-<br />

10E.<br />

Attenuating mechanisms between seeps SP- 19, SP- I OE and SP- 1 OW include:<br />

Precipitation of aluminum hydroxides (implied by MMTEQA2 and field observations)<br />

Precipitation of ferric hydroxides (implied by MINTEQA2 and field observations)<br />

Co-precipitation of copper and zinc with ferric hydroxides (implied by observations from<br />

elsewhere that indicate ferric hydroxide precipitates also contain these metals)<br />

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17693-005-019Uuly 27. lm4: 1 1 PM;DRAFT FINAL RI REPORT<br />

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